Hydraulic apparatus

ABSTRACT

This invention provides a variable-positive-displacement pump such as a swashplate pump having a servomotor to control pump displacement. The pump displacement is urged by a constantly acting biasing force towards zero displacement and the servomotor includes one working space within which pressure acts to tend to increase pump displacement. The pressure in the working space is that at the junction of a pair of restrictors connected in series between the pump delivery and a low-pressure zone. The restrictor connected to the low-pressure zone is adjustable to vary the pressure in the servomotor and thus to vary selected displacement. When the servomotor is close to the zero displacement position the restrictive effect of the two restrictors in series is reduced so as to provide a passage through which the delivery of the pump may flow without generating a substantial pressure.

United States Patent [72] lnventor Kenneth Raymond Boydell Bredons Hardwick, near Tewkesbury, England [2!] Appl. No. 808,780

[22] Filed Mar. 20, 1969 [4S] Patented June 29, 1971 73] Assignee Dowty Technical Developments Limited Cheltanham, England [32] Priority Mar. 27, 1968 [3 3] Great Britain [54] HYDRAULIC APPARATUS 2,284,897 6/1942 Harrington Primary Examiner-William L. Freeh AttorneyYoung & Thompson ABSTRACT: This invention provides a variable-positive-displacement pump such as a swashplate pump having a servomotor to control pump displacement. The pump displacement is urged by a constantly acting biasing force towards zero displacement and the servomotor includes one working space within which pressure acts to tend to increase pump displacement. The pressure in the working space is that at the junction of a pair of restrictors connected in series between the pump delivery and a low-pressure zone. The restrictor connected to the, low-pressure zone is adjustable to vary the pressure in the servomotor and thus to vary selected displacement. When the servomotor is close to the zero displacement position the restrictive effect of the two restrictors in series is reduced so as to provide a passage through which the delivery of the pump may flow without generating a substantial pressure.

HYDRAULIC APPARATUS This invention relates to a-variable-positive-displacement pump having a servomotor connected to adjust its displacement.

The present invention according to one aspect provides a variable-positive-displacement pump, a servomotor for adjusting the pump displacement, means biasing the servomotor in a pump-displacement-reducing direction, and a control system for the servomotor including a pair of restrictors connected in series between a low-pressure zone and the delivery connection of the pump, the pressure in the liquid flow passage between the two restrictors acting on the servomotor in a pump-displacement-increasing direction, means for adjusting the restrictor connected to the zone of low-pressure to vary the pressure in the passage, and means arranged to reduce the restrictive effect of the other restrictor as pump displacement approaches the minimum displacement position whereby to unload the pump through the restrictor connected to the zone of low-pressure.

The means for adjusting the restrictor connected to the zone of low-pressure may respond both to the movement of a control member and to the movement of the movable servomotor member in such manner that movement of the movable servomotor member cancels the adjustment given to that restrictor by the control member.

The delivery connection of the pump may be joined to its hydraulic load through the medium of a valve adapted to be closed when minimum pump displacement is selected.

The present invention according to another aspect provides a variable-positivedisplacement pump, a servomotor for adjusting the pump displacement, means biasing the servomotor in a pump-displacement reducing direction and a control system for the servomotor including a pair of restrictors connected in series between a low-pressure zone and the delivery connection of the pump, the pressure in the liquid flow passage between the two restrictors acting on the servomotor in a pump-displacement-increasing direction means for adjusting the restrictor connected to the zone of low-pressure pressure to vary the pressure in the passage and means to reduce the overall restrictive effect of these restrictors between the delivery connection of the pump and the lowpressure zone as pump displacement approaches the minimum displacement position whereby to unload the pump.

The phase unload the pump means the connection to the pump delivery connection of a flow path through which the whole delivery of the pump may pass without generation of substantial pressure whereby very little energy is necessary to drive the pump. The unloading of the pump also provides a circulation of liquid through the pump whilst it is being driven which will dissipate heat generated in the pump.

Two embodiments of the invention will now be particularly described with reference to the accompanying drawings in which,

FIG. 1 is a diagrammatic view showing a part of the pump 4 and the servomotor, and

FIG. 2 is a cross section through a modified form of servomotor.

Reference is made initially to FIG. 1 of the accompanying drawing. The pump indicated at 1 is constructed substantially in accordance with the drawings of the complete specification filed in pursuance of our application Ser. No. 694717. Basically this pump includes a valve block 2, a casing 2 and lugs formed on the casing to carry a swashplate. The valve block 2, casing 3 and lugs are integrally formed. A lever 6 extends from the swashplate. A servomotor 7 is secured to the valve block by any convenient means. The servomotor casing comprises a fixed cylinder 8 within which a hollow piston 9 is slidably mounted. A cylindrical connecting rod 11 formed integrally with the piston extends through a sealed opening 12 in the end of the cylinder 8 nearer to the lever 6. The end of the connecting rod 11 is pivotally connected by a pin 13 to a link 14 which in turn is pivotally connected by pin 15 to the end of the lever 6. Within the cylinder 8 a working space 16 is defined between the piston 9, the cylinder 8, the connecting rod 11 and the sealed opening 12 in the :end of the cylinder. Liquid at pressure from the delivery connection of the pump 1 is fed to connection 17 of the cylinder 8 and liquid may flow to the working space 16 from the connection 17 through a fixed orifice 18 forming the restrictor connected to the delivery connection of the pump.

The position of the lever 6 indicated is the minimum displacement position which however is not quite the zero displacement for the pump. At this illustrated position a groove 19 formed around the connecting rod 11 is arranged to make connection between the working space 16 and a small diameter passage 21 extending from the connection 17 to the opening 12. The passage 21 forms a bypass orifice.

The control for the servomotor comprises a hollow cylindrical slide valve 22 mounted within a cylindrical bore 23 in the piston rod 11 adjacent to the piston 9. A port 24 in the connecting rod 11 connects the bore 23 to the working space 16. The piston valve 22 also includes a port 25 which communicates with its central bore 26. The bore 26 opens through a drilling 27 into the interior of the hollow piston, from which access is obtained through the open end 28 of the cylinder to reservoir. The piston valve 22 is mechanically connected to the piston 9 by means of a cross pin 29 which is a tight fit within the piston valve but is capable of longitudinal movement within elongated holes 31 in the piston. A flexible cable 32 controlled manually for example as shown in application Ser. No. 808,781 of even date herewith, is secured to pin 29 to adjust valve 22.

in the midposition of the pin 29 within the lost motion permitted in the holes 31 there is a small overlap as shown in between the ports 24 and 25 which form the restrictor connected to the zone of low-pressure whose restrictive effect is about the same as the orifice 18. In order to increase the displacement of the pump, the cable 32 is pulled to the left as seen in the drawing to move the pin 29 to the end of its lost motion in the holes 31. This movement will reduce the overlap between port 24 and 25 thus increasing the restrictive effect, and the pressure in the working space 16 will thereby increase towards the value of pressure supplied at connection 17. This pressure reacting on the annular area of piston 9 exposed to working space 16 will cause piston 9 and connecting rod 1 1 to move to the left as seen in the drawing to shorten the overall length of the servomotor and to increase the tilt of the swashplate. lf tension is relaxed on the cable 32 a slight further movement of the piston 9 will take it to the position where the pin 29 is midway within the lost motion given by holes 31 and the ports 24 and 25 will increase their overlap thus reducing the pressure in the working space 16. The pump is a swashplate pump in which the pivot of the swashplate is deliberately offset from the center line of the rotating pump cylinder block so that there will be a considerable restoring force on the swashplate as a result of hydraulic pressure within the pump, such restoring force being arranged to move the swashplate towards its minimum displacement condition and at the same time to move the servo piston to increase the overall length of the servomotor. Suitable stops are arranged for the swashplate to engage it at its minimum displacement position so that the hydraulic force exerted within the pump on the swashplate cannot move the swashplate through zero displacement into a reverse displacement position. The working space 16 also forms the flow passage between the two restrictors.

At the selected value for pump displacement the overlap between the ports 24 and 25 will be self-adjusting by slight movement of the piston 9 to adjust pressure in the working space 16 so that the force on the connecting rod 11 is exactly equal and opposite to the force exerted by lever 6 on pin 15. In order to reduce selected displacement the flexible cable 32 is pushed into the open end of the cylinder so that the spring 63 may move the piston valve 22 to give a greater overlap between ports 24 and 25. lt will be appreciated that the piston valve 22 is hydraulically balanced and it needs very little force to urge it into the bore 23. The greater overlap of the ports 24 and 25 effectively reduces the pressure in the working space 16 and enables the self-restoring force of the swashplate to reduce the selected displacement for the pump.

As the pump approaches closely to its minimum displacement position, the groove 19 will overlap the end of passage 21 in bore 12 making a restricted bypass connection from connection 17 into the working space 16 in parallel with the restricted connection already provided by orifice 18. The passage 21 then provides a bypass orifice parallel with the orifice 18 which reduced the total restrictive effect of the restrictor between the pump delivery connection and the working space 16. From the working space 16 liquid will of course have access through the overlapping ports 24 and 25 to a reservoir. The small delivery given by the pump at its minimum displacement position may then flow through con nection 17, the parallel orifices l8 and 21, working space 16 and ports 24 and 25 back to a reservoir. The pressure generated in the pump will therefore only be slight and the pump will therefore be effectively unloaded. In order to increase the displacement of the pump movement is given to the piston valve 22 to close or nearly close the restrictor formed by ports 24 and 25 thereby enabling the pump to generate a small pressure which will move the servo piston.

The delivery connection of the pumpis connected to a hydraulic load 41 through the medium of a reversing valve 42 and a pair of pipes 43 and 44. The valve 42 includes a hand lever 45 adjustable into a forward, a neutral, and a reverse position. In the neutral position the valve 42 closes the delivery connection 5. In the forward and reverse positions the delivery connection 5 is connected respectively to the pipe 43 or 44 so as to cause movement of the hydraulic motor 41 in one direction or the other, return flow from the motor being through the other pipe 44 or 43 and pipe 47 to reservoir. The handle 45 may be connected also for operation of the valve 22 as shown in said application Ser. No. 808,781.

Reference is now made to FIG. 2 of the accompanying drawings which illustrates a modified form of the servomotor shown in FIG. 1 and similar reference numerals will be used to indicate parts having similar functions. In FIG. 2 the main difference in construction is to cause the servomotor to effect a pushing force as a result of hydraulic pressure fed into its working space 16. Lever 6 extending from the swashplate is illustrated in the position corresponding to minimum displacement and to increase displacement lever 6 will mover to the right as seen in the drawing. The tilt axis of the swashplate for this embodiment is oppositely offset to that in FIG. 1 so that the effect of hydraulic pressure within the pump is to urge the lever 6 to the left as shown in FIG. 2. The pump delivery connection 17 feeds liquid at pressure from the pump to the restrictor 18 from which liquid flows to the working space 16 which forms the flow passage between the two restrictors. The bypass orifice 21 in parallel with the orifice 18 is now comprised by a groove milled in the valve 22, this groove being so arranged that when the piston 9 is in its position corresponding to minimum pump displacement the orifice 21 will join connection 17 to working space 16. The restrictor connected to the zone of low-pressure is formed by the overlap between the port 25 in valve 22 and the edge of the piston in the working space 16, and controls escape flow of liquid from working space 16 through bore 26 in valve 22 to the open end of the servo cylinder 8 which effectively connects to reservoir. A transverse pin 2 secured in the piston passes loosely through a hole 31 in valve 22 defining the lost motion of the valve relative to the piston. In this embodiment the manual control of the valve 22 is effected by a lever 85 extending from a rotary shaft 86 mounted in the servomotor body 7. The valve 22 carries a flange in the form of a circlip 87 which engages the servomotor body to form a positive stop on valve movement at the minimum displacement position of the pump.

In order' to increase pump displacement valve 22 is moved to the right by lever 85 to cause closure of the restrictor formed by orifice 25. The pressure fed to the working space 16 will therefore rise and the piston 9 will be urged to the right causing the required increase in pump displacement. Piston movement will follow the adjustment of the valve 22 since piston movement relative to valve 22 is always in a sense to adjust the restrictor formed by port 25 to adjust the pressure in the working space 16 to balance exactly the restoring force exerted by the swashplate 6. In order to produce pump displacement the valve 22 is moved to the left to open the restrictor formed by port 25 thus reducing pressure in the working space 16 and effectively unloading the pump. The force acting on the swashplate 6 will then move the piston 9 to the left and reduction of displacement will take place to the amount dictated by the movement of the valve 22.

Whilst the illustrated embodiments of the invention arrange that the restrictor connected to the zone of low-pressure is adjustable both by movement of the manual control and by movement of the servomotor to provide a followup effect, it will be appreciated that within this scope of the present invention that restrictor may be adjusted independently of the servomotor so that a followup effect is not obtained.

Whilst in the illustrated embodiments the restrictor connected to the delivery connection of the pump has a fixed restrictive effect through the major part of pump displacement adjustment it is within the scope of the present invention for pump displacement adjustment to be effected by opposite variations of the restrictive effects of the two restrictors so that during movement towards zero displacement the restrictor connected to the zone of low-pressure will have its restrictive effect reduced and the restrictor connected to the delivery connection of the pump will have its restrictive effect increased. In this arrangement according to our invention as pump displacement approaches the minimum displacement position the restrictive effect of the restrictor connected to the delivery connectionof the pump will be reduced in order to unload the pump through the restrictor connected to the zone of low-pressure.

lclaim:

1. A variable-positive-displacement pump, a servomotor for adjusting the pump displacement, means biasing the servomotor in a pump-displacement-reducing direction, and a control system for the servomotor including a pair of restrictor means connected in series between a low-pressure zone and the delivery connection of the pump, the pressure in the liquid flow passage between the two restrictor means acting on the servomotor in a pump-displacment-increasing direction, means for adjusting the restrictor means which is connected to the zone of low-pressure to vary the pressure in the passage whereby reduction in its restrictive effect will reduce pump displacement and vice versa, and means to increase the total cross section of the flow passage of the other restrictor means as pump displacement approaches the minimum displacement position whereby in the minimum displacement position the pump delivery may flow through the two restrictor means in series when both have a minimum restrictive effect, thus unloading the pump.

2. A variable-positive-displacement pump according to claim 1 wherein the means for adjusting the restrictor means connected to the zone of low-pressure responds to both to the movement of a control member and to the movement of the movable servomotor member in such manner that movement of the movable servomotor member cancels the adjustment given to that restrictor by the control member.

3. A variable-positive-displacement pump according to claim 1 wherein the delivery connection of the pump is joined to a hydraulic load through the medium of a valve adapted to be closed when minimum pump displacement is selected.

4. A variable-positive-displacement pump according to claim 1 wherein the said other restrictor means comprises a fixed orifice, and the means for reducing the restrictive effect provided by this orifice comprises a bypass orifice connected across the fixed orifice by movement of the pump displacement adjusting mechanism as it approaches its minimum displacement position.

5. A v'ariable-positive-displacement pump, a servomotor for adjustingthe pump displacement, means biasing the servomotor in a pump-displacement-reducing direction and a control system for the servomotor including a pair of restrictor means connected in series between a low-pressure zone and the delivery connection of the pump, the pressure in the liquid flow passage between the two restrictor means acting on the servomotor in a pump-displacement-increasing direction, means for adjusting the restrictor means which is connected to the zone of low-pressure to vary the pressure in the passage 

1. A variable-positive-displacement pump, a servomotor for adjusting the pump displacement, means biasing the servomotor in a pump-displacement-reducing direction, and a control system for the servomotor including a pair of restrictor means connected in series between a low-pressure zone and the delivery connection of the pump, the pressure in the liquid flow passage between the two restrictor means acting on the servomotor in a pump-displacmentincreasing direction, means for adjusting the restrictor means which is connected to the zone of low-pressure to vary the pressure in the passage whereby reduction in its restrictive effect will reduce pump displacement and vice versa, and means to increase the total cross section of the flow passage of the other restrictor means as pump displacement approaches the minimum displacement position whereby in the minimum displacement position the pump delivery may flow through the two restrictor means in series when both have a minimum restrictive effect, thus unloading the pump.
 2. A variable-positive-displacement pump according to claim 1 wherein the means for adjusting the restrictor means connected to the zone of low-pressure responds to both to the movement of a control member and to the movement of the movable servomotor member in such manner that movement of the movable servomotor member cancels the adjustment given to that restrictor by the control member.
 3. A variable-positive-displacement pump according to claim 1 wherein the delivery connection of the pump is joined to a hydraulic load through the medium of a valve adapted to be closed when minimum pump displacement is selected.
 4. A variable-positive-displacement pump according to claim 1 wherein the said other restrictor means comprises a fixed orifice, and the means for reducing the restrictive effect provided by this orifice comprises a bypass orifice connected across the fixed orifice by movement of the pump displacement adjusting mechanism as it approaches its minimum displacement position.
 5. A variable-positive-displacement pump, a servomotor for adjusting the pump displacement, means biasing the servomotor in a pump-displacement-reducing direction and a control system for the servomotor including a pair of restrictor means connected in series between a low-pressure zone and the delivery connection of the pump, the pressure in the liquid flow passage between the two restrictor means acting on the servomotor in a pump-displacement-increasing direction, means for adjusting the restrictor means which is connected to the zone of low-pressure to vary the pressure in the passage whereby reduction in its restrictive effect will reduce pump displacement and vice versa, and means to increase the total cross section of the flow passage of these restrictor means between the delivery connection of the pump and the low-pressure zone as pump displacement approaches the minimum displacement position whereby in the minimum displacement position the pump delivery may flow through the two restrictor means in series when both have a minimum restrictive effect, thus unloading the pump. 